Coatings and powders, methods of making same, and uses thereof

ABSTRACT

This invention relates to thermal spray coatings, powders useful in deposition of the thermal spray coatings, methods of producing the powders, and uses of the thermal spray coatings, for example, coating of piston rings and cylinder liners of internal combustion engines. The coatings of this invention are applied by thermal spray deposition of a powder. The powder contains bimetallic carbides of chromium and molybdenum dispersed in a matrix metal. The matrix metal contains nickel/chromium/molybdenum.

FIELD OF THE INVENTION

This invention relates to thermal spray coatings, powders useful indeposition of the thermal spray coatings, methods of producing thepowders, and uses of the thermal spray coatings, for example, coating ofpiston rings and cylinder liners of internal combustion engines.

BACKGROUND OF THE INVENTION

As new technologies continue to be developed resulting in internalcombustion engines, compressors, etc. having increasingly higherperformance criteria such as higher power generation, it has requiredthat certain components of the internal combustion engines, compressors,etc., especially the surfaces of components in sliding contact withsurfaces of other components such as piston rings and cylinder liners,have increasingly better wear, scuffing, peeling and corrosionresistance.

The outer peripheral surfaces of piston rings and in some cases cylinderliners made of cast iron or steel have been subjected to various surfacetreatments over the years. Such treatments have included, for example,nickel composite plating, hard chromium plating, thermal spraytreatments, and the like.

For example, U.S. Pat. No. 6,562,480 B1 discloses a wear resistantcoating for protecting a surface undergoing sliding contact with anothersurface such as piston rings and cylinder liners of internal combustionengines. The wear resistant coating is applied by HVOF deposition of apowder which comprises a blend of about 13 weight percent to about 43weight percent of a nickel-chromium alloy, about 25 weight percent toabout 64 weight percent chromium carbide, and about 15 weight percent toabout 50 weight percent molybdenum.

While some of the surface treatments have been found to be suitable forcertain applications, the newer technologies require improved surfacetreatments that provide excellent wear, scuffing, peeling and/orcorrosion resistance, in particular for applications involving surfacesof components in sliding contact with surfaces of other components suchas piston rings and cylinder liners. Current surface treatments havebeen found to be inadequate in one or more properties for the newertechnologies.

Especially for the newer internal combustion engine, e.g., diesel, andcompressor technologies having increasingly higher performance criteria,a need exists for powders and coatings that can be deposited by thermalspray devices and that exhibit excellent wear, scuffing, peeling and/orcorrosion resistance. A need exists for developing new powders and forthermal spray deposition of wear, scuffing, peeling and corrosionresistant coatings for components of the internal combustion engines,compressors, etc. It would be desirable in the art to provide powdersand coatings that can be deposited by thermal spray devices and thatexhibit excellent wear, scuffing, peeling and corrosion resistance forcomponents of the internal combustion engines, compressors, etc.

SUMMARY OF THE INVENTION

This invention relates in part to a powder useful for deposition througha thermal spray device, said powder comprising bimetallic carbides ofchromium and molybdenum dispersed in a matrix metal, said matrix metalcomprising nickel/chromium/molybdenum.

This invention also relates in part to a process for preparing a powderuseful for deposition through a thermal spray device, said powdercomprising bimetallic carbides of chromium and molybdenum dispersed in amatrix metal, said matrix metal comprising nickel/chromium/molybdenum;wherein said process comprises (i) providing a blend of Cr3C2, NiCr andMoOx (wherein x is a value of from about 0 to about 3), (ii) subjectingsaid blend to a reducing atmosphere, e.g., hydrogen, under conditionssufficient to produce said bimetallic carbides of chromium andmolybdenum, and (iii) dispersing said bimetallic carbides of chromiumand molybdenum in said matrix metal.

This invention further relates in part to a coating deposited by athermal spray device, said coating comprising bimetallic carbides ofchromium and molybdenum dispersed in a matrix metal, said matrix metalcomprising nickel/chromium/molybdenum.

This invention yet further relates in part to a coating applied bythermal spray deposition of a powder, said powder comprising bimetalliccarbides of chromium and molybdenum dispersed in a matrix metal, saidmatrix metal comprising nickel/chromium/molybdenum.

This invention also relates in part to a piston ring having a wearresistant coating deposited by a thermal spray device, said coatingcomprising bimetallic carbides of chromium and molybdenum dispersed in amatrix metal, said matrix metal comprising nickel/chromium/molybdenum.

This invention further relates in part to a piston ring having a wearresistant coating applied by thermal spray deposition of a powder, saidpowder comprising bimetallic carbides of chromium and molybdenumdispersed in a matrix metal, said matrix metal comprisingnickel/chromium/molybdenum.

The thermal spray coatings of this invention are useful in a variety ofapplications. In addition to coatings for piston rings and cylinderliners of internal combustion engines, the coatings of this inventionare useful for extending the life of components in environmentsrequiring wear resistance, corrosion resistance and/or high temperatureresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of sand abrasion for the materialsidentified in Examples 1 and 2. FIG. 1 shows the material volume removedby sand in mm³ per 1,000 revolutions. As shown in FIG. 1, less volumeloss represents a more wear resistant material.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, this invention relates to wear resistant powdersuseful for deposition through thermal spray devices such as plasma, HVOFor detonation gun. The powders contain bimetallic carbides of chromiumand molybdenum dispersed in a matrix metal. The matrix metal containsnickel/chromium/molybdenum. The powders are useful for forming wearresistant coatings having the same composition.

The powders of this invention contain from about 45 to about 75 weightpercent chromium, from about 8 to about 35 weight percent molybdenum,from about 10 to about 30 weight percent nickel, and from about 3 toabout 8 weight percent carbon. Preferably, the powders of this inventioncontain from about 50 to about 70 weight percent chromium, from about 12to about 30 weight percent molybdenum, from about 15 to about 25 weightpercent nickel, and from about 3.5 to about 6.5 weight percent carbon.

The powders of this invention comprise agglomerated particles. Theparticles are comprised of bimetallic carbides of chromium andmolybdenum dispersed in a matrix metal. The matrix metal containsnickel/chromium/molybdenum. The powder particles have an averageparticle size (agglomerated) of from about 5 micrometers to about 55micrometers, preferably from about 15 micrometers to about 50micrometers, and more preferably from about 20 micrometers to about 45micrometers. The average particle size of the thermal spraying powdersis preferably set according to the type of thermal spray device andthermal spraying conditions used during thermal spraying.

The bimetallic carbides of chromium and molybdenum have an averageparticle or grain size of less than about 8 microns, preferably lessthan about 4 microns, and more preferably less than about 2 microns.These particles are dispersed in a metal matrix to form the agglomeratedparticles described above.

The powders of this invention can contain a large volume of bimetalliccarbides of chromium and molybdenum for imparting excellent wearresistance to coatings made therefrom. Powders of this inventioncontaining less than about 45 weight percent chromium may exhibitinadequate wear and corrosion resistance for many applications. Chromiumlevels in excess of about 75 weight percent may tend to detract from thewear resistance of the coating because the coating may become toobrittle. Powders containing less than about 8 weight percent molybdenummay exhibit inadequate wear resistance for many applications. Molybdenumlevels in excess of about 35 weight percent may tend to detract from thewear resistance of the coating because the coating may become brittleand lead to increased peeling.

The carbon concentration controls the volume percent of carbide phases.A large volume percent of carbide phases increases the hardness andimproves the wear properties of coatings formed with the powders. Aminimum of about 3 weight percent carbon may be necessary to impartadequate hardness into the coatings. If the carbon exceeds about 8weight percent, the volume percent of carbide phases may become too highcausing brittleness.

Increasing the concentration of nickel may tend to increase thedeposition efficiency for thermal spraying the powders. Because totalnickel levels above about 30 weight percent may tend to soften thecoating and limit the wear resistance of the coating, the totalconcentration of nickel may best be maintained above about 10 weightpercent and below about 30 weight percent.

The thermal spraying powders useful in this invention can be produced byagglomeration (spray dry and sinter or sinter and crush methods). In aspray dry and sinter method, a slurry is first prepared by mixing aplurality of raw material powders and a suitable dispersion medium. Thisslurry is then granulated by spray drying, and a coherent powderparticle is then formed by sintering the granulated powder. The thermalspraying powder is then obtained by sieving and classifying (ifagglomerates are too large, they can be reduced in size by crushing).The sintering temperature during sintering of the granulated powder ispreferably 1000 to 1300° C.

In the sinter and crush method, a compact is first formed by mixing aplurality of raw material powders followed by compression and thensintered at a temperature between 1100 to 1400° C. The thermal sprayingpowder is then obtained by crushing and classifying the resultingsintered compact into the appropriate particle size distribution.

If the average particle size of a raw material powder is too small, rawmaterial costs may be prohibitive. If the average particle size of a rawmaterial powder is too large, it may become difficult to uniformlydisperse the raw material powder. The raw material particle size ispreferably no greater than about 10 microns.

The individual particles that compose the thermal spraying powderpreferably have enough mechanical strength to stay coherent during thethermal spraying process. If the mechanical strength is too small, thepowder particle may break apart clogging the nozzle or accumulate on theinside walls of the thermal spray device.

In particular, methods of making the thermal spray powders of thisinvention can include CrC/NiCr+MoOx (reduce) processes and CrC+NiCr+MoOx(reduce) processes (wherein x is a value of from about 0 to about 3).The CrC/NiCr+MoOx (reduce) process is carried out in Example 1hereinbelow. This process involves taking CrC/NiCr (already agglomeratedand sintered) and adding MoOx followed by reduction (sintered for asecond time). The CrC+NiCr+MoOx (reduce) process can involve fewer stepsby starting with 3 components, i.e., CrC+NiCr+MoOx, and then sinteringfollowed by reduction, thereby eliminating a sintering step.

Alternatively, the powders of this invention may be produced by means ofinert gas atomization of a mixture of elements in the proportions statedherein. Preferred atomization methods that may be employed in making thepowders of this invention are described in U.S. Pat. No. 5,863,618, thedisclosure of which is incorporated herein by reference. The rawmaterials of these powders are typically melted at a temperature ofabout 1600° C. and then atomized in a protective atmosphere (e.g.,argon, helium or nitrogen). Most advantageously the atmosphere is argon.A nitrogen atmosphere may be employed which may result in the formationof additional hard phases interspersed throughout the alloys, e.g.,nitrides. As indicated above, to facilitate melting for atomization, thealloy may optionally contain melting point suppressants like boron,silicon and manganese.

Preferably, the thermal spraying powders useful in this invention can beprepared by (i) providing a blend of Cr3C2, NiCr and MoOx (wherein x isa value of from about 0 to about 3), (ii) subjecting the blend to areducing atmosphere, e.g., hydrogen, under conditions sufficient toproduce bimetallic carbides of chromium and molybdenum, and (iii)dispersing the bimetallic carbides of chromium and molybdenum in amatrix metal. The blend of Cr3C2, NiCr and MoOx, e.g., Cr3C2/NiCr andMoOx, can be reduced in a hydrogen pusher furnace and then furtherreduced in a vacuum furnace. Hydrogen reduces some of the oxide (formingH2O) but carbon from the chromium carbide can also reduce the molybdenumoxide (forming CO/CO2). The resulting carbides are bimetallic carbidesof chromium and molybdenum. Illustrative bimetallic carbides of chromiumand molybdenum include, for example, MC, M2C, M3C2, M7C3, M23C6, M6C,(wherein M is chromium and molybdenum) and the like.

Coatings may be produced using the powders of this invention by avariety of methods well known in the art. These methods include thermalspray (plasma, HVOF, detonation gun, etc.), laser cladding; and plasmatransferred arc (PTA). Thermal spray is a preferred method fordeposition of powders to form the coatings of this invention. Thepowders of this invention are useful for forming wear resistant coatingshaving the similar composition.

The coating process involves flowing powder through a thermal sprayingdevice that heats and accelerates the powder onto a substrate. Uponimpact, the heated particle deforms resulting in a thermal sprayedlamella or splat. Overlapping splats make up the coating structure. Adetonation process is disclosed in U.S. Pat. No. 2,714,563, thedisclosure of which is incorporated herein by reference. The detonationprocess is further disclosed in U.S. Pat. Nos. 4,519,840 and 4,626,476,the disclosures of which are incorporated herein by reference. U.S. Pat.No. 6,503,290, the disclosure of which is incorporated herein byreference, discloses a high velocity oxygen fuel (HVOF) process.

The coatings of this invention contain from about 45 to about 75 weightpercent chromium, from about 8 to about 35 weight percent molybdenum,from about 10 to about 30 weight percent nickel, and from about 3 toabout 8 weight percent carbon. Preferably, the coatings of thisinvention contain from about 50 to about 70 weight percent chromium,from about 12 to about 30 weight percent molybdenum, from about 15 toabout 25 weight percent nickel, and from about 3.5 to about 6.5 weightpercent carbon.

The coatings of this invention can contain a large volume of chromiumand molybdenum for imparting excellent wear resistance to coatings madetherefrom. Coatings containing less than about 45 weight percentchromium may exhibit inadequate wear and corrosion resistance for manyapplications. Chromium levels in excess of about 75 weight percent maytend to detract from the wear resistance of the coating because thecoating may become too brittle. Coatings containing less than about 8weight percent molybdenum may exhibit inadequate wear resistance formany applications. Molybdenum levels in excess of about 35 weightpercent may tend to detract from the wear resistance of the coatingbecause the coating may become brittle and lead to increased peeling.

The carbon concentration controls the volume percent of carbide phases.A large volume percent of carbide phases increases the hardness andimproves the wear properties of coatings. A minimum of about 3 weightpercent carbon may be necessary to impart adequate hardness into thecoatings. If the carbon exceeds about 8 weight percent, the volumepercent of carbide phases may become too high causing brittleness.

Increasing the concentration of nickel may also tend to increase thedeposition efficiency by thermal spraying. Because total nickel levelsabove about 30 weight percent may tend to soften the coating and limitthe wear resistance of the coating, the total concentration of nickelmay best be maintained above about 10 weight percent and below about 30weight percent.

The thickness of the thermal spray coatings of this invention, forexample, the thickness of the coating formed on an outer peripheralsurface of the piston ring, is typically from about 25 micrometers toabout 500 micrometers, preferably from about 50 micrometers to about 250micrometers, and more preferably from about 60 micrometers to about 140micrometers. When the thickness of the thermal spray coating is lessthan about 25 micrometers, the piston ring fails to achieve apredetermined life expectancy. When the thickness of the thermal spraycoating exceeds about 500 micrometers, the coating may exhibit excessiveresidual stresses causing spalling or peeling.

A coated piston ring is typically used in combination with a cylinderliner (e.g., cast iron). The piston ring is typically made of steelssuch as carbon steel, low alloy steel, or cast iron such as spheroidalgraphite cast iron. To reduce abradability on the cylinder liner, thebimetallic carbides of chromium and molybdenum (dispersed in the matrixmetal) have an average particle or grain size of less than about 8microns, preferably less than about 4 microns, and more preferably lessthan about 2 microns. When the average particle size of the bimetalliccarbides of chromium and molybdenum exceeds about 8 microns, theparticles function as abrasive grains resulting in greater wear of thecylinder liner.

When the bimetallic carbides of chromium and molybdenum function asabrasive grains projecting from the surface of the coating or freeabrasive grains debonded from the coating, the piston ring wears orabrades the cylinder liner. The bimetallic carbides of chromium andmolybdenum preferably have fine size to prevent them from functioning asabrasive grains. Preferably, the bimetallic carbides of chromium andmolybdenum are round or have soft edges.

The coatings of this invention can have a porosity of from about 0.2percent to about 5 percent by volume, preferably from about 0.4 percentto about 3 percent by volume, and more preferably from about 0.5 percentto about 1.5 percent by volume, based on the entire thermal spraycoating. When the porosity exceeds about 5 percent by volume, the wearresistance decreases.

The coatings of this invention preferably have sufficientmicrostructure, hardness and wear resistance such that the coatings cansuppress the wear of the cylinder liners. The average hardness of thethermal spray coatings of this invention is at least about 500 Hv0.1,preferably at least about 650 Hv0.1, and more preferably at least about800 Hv0.1. The hardness of the thermal spray coatings is expressed byVickers hardness according to JIS Z 2244.

The coatings of this invention have a surface roughness sufficient toprevent wear of the mating member or cylinder liner by sliding. Thecoating surface is finished to provide the smooth sliding surface. Thefinished sliding surfaces have a surface roughness less than about 1.0micrometers, preferably less than about 0.5 micrometers, and morepreferably less than about 0.1 micrometers. The surface roughness isbased on arithmetic mean average roughness Ra. When the surfaceroughness exceeds about 1.0 micrometers, the piston ring is moreabrasive on the cylinder liner.

Other ingredients may be added to the coatings of this invention. Forexample, because ceramic powders such as tungsten carbide have highmelting points and high hardness, they may be added to improve wearresistance.

The powders of this invention are useful for forming coatings on objectshaving excellent wear, corrosion and high temperature oxidationproperties, for example, coatings for protecting surfaces undergoingsliding contact with other surfaces such as piston rings and cylinderliners of internal combustion engines.

The examples that follow are intended as an illustration of certainpreferred embodiments of the invention, and no limitation of theinvention is implied.

Example 1

A standard CrC/NiCr, spray dried and sintered, thermal spray powder(Praxair 1375VM) was blended with MoO3. This powder mix was heated in ahydrogen furnace to reduce the MoO3. The resulting molybdenum (Mo) wasfound in both the carbide grains and the metallic matrix. This powderwas separated into an appropriate thermal spray size (10-50 μm) andsprayed by JP-5000 (HVOF) to form a thermal spray coating upon a steelsubstrate. Standard spray parameters were utilized for CrC/NiCrcoatings. The surface of the coating sample was ground to 0.5 μm Ra andused to evaluate the abrasion resistance of the coating by ASTM G-65(dry sand, rubber wheel). The sand abrasion resistance is set forth inTable A below.

Example 2

For comparison purposes, two other samples were prepared. A standardCrC/NiCr, spray dried and sintered, thermal spray powder (Praxair1375VM) was sprayed by JP-5000 (HVOF). Standard spray parameters wereutilized for CrC/NiCr coatings. Additionally, this same CrC/NiCr powderwas used again to make a simple blend of CrC/NiCr plus 20% (by weight)molybdenum (1375VM+Mo). These two comparison samples were also ground toapproximately 0.5 μm Ra and were evaluated by ASTM G-65 (dry sand,rubber wheel) to measure their resistance to abrasion. Table A setsforth the wear rate of each sample (material volume removed by sand inmm³ per 1,000 revolutions). The wear rate for a chromium plated sample(as a reference) is also set forth in Table A.

TABLE A Material Scar Volume Cr-Plate 8.3 1375VM + Mo 4.3 1375VM 3.9Example 1 2.6In Table A, the Example 1 coating shows less volume loss whichrepresents more resistance to abrasion.

Other variations and modifications of this invention will be obvious tothose skilled in the art. This invention is not limited except as setforth in the claims.

The invention claimed is:
 1. A powder useful for deposition through athermal spray device, said powder comprising bimetallic carbides ofchromium and molybdenum dispersed in a matrix metal, said matrix metalbased on nickel/chromium/molybdenum, wherein the powder furthercomprises a carbon concentration of greater than 3.5 wt % but no greaterthan 6.5 wt %.
 2. The powder of claim 1 which comprises from about 45 toabout 75 weight percent chromium, from about 8 to about 35 weightpercent molybdenum, from about 10 to about 30 weight percent nickel. 3.The powder of claim 1 which comprises from about 50 to about 70 weightpercent chromium, from about 12 to about 30 weight percent molybdenum,from about 15 to about 25 weight percent nickel.
 4. The powder of claim1 comprising powder particles, said powder particles comprisingbimetallic carbides of chromium and molybdenum dispersed in a matrixmetal, said matrix metal based on nickel/chromium/molybdenum, saidpowder particles having an average particle size (agglomerated) of fromabout 5 micrometers to about 55 micrometers.
 5. The powder of claim 1comprising powder particles, said powder particles comprising bimetalliccarbides of chromium and molybdenum dispersed in a matrix metal, saidmatrix metal based on nickel/chromium/molybdenum, said powder particleshaving an average particle size (agglomerated) of from about 15micrometers to about 50 micrometers.
 6. The powder of claim 1 whereinsaid bimetallic carbides of chromium and molybdenum have a particle orgrain size of less than about 8 microns.
 7. The powder of claim 1wherein said bimetallic carbides of chromium and molybdenum have aparticle or grain size of less than about 4 microns.
 8. The powder ofclaim 1 wherein said thermal spray device is selected from detonationgun, high velocity oxygen fuel (HVOF) and plasma.
 9. A process forpreparing a powder useful for deposition through a thermal spray deviceaccording to claim 1, said powder comprising bimetallic carbides ofchromium and molybdenum dispersed in a matrix metal, said matrix metalbased on nickel/chromium/molybdenum; wherein said process comprises (i)providing a blend of Cr₃C₂, NiCr and MoOx (wherein x is a value of fromabout 0 to about 3), (ii) subjecting said blend to a reducing atmosphereunder conditions sufficient to produce said bimetallic carbides ofchromium and molybdenum, and (iii) dispersing said bimetallic carbidesof chromium and molybdenum in said matrix metal.
 10. The process ofclaim 9 wherein the blend comprises agglomerated Cr₃C₂/NiCr and MoOx.11. The process of claim 9 wherein the reducing atmosphere is created ina hydrogen pusher furnace and/or in a vacuum furnace.
 12. The process ofclaim 9 wherein said powder comprises from about 45 to about 75 weightpercent chromium, from about 8 to about 35 weight percent molybdenum,from about 10 to about 30 weight percent nickel, and from about 3 toabout 8 weight percent carbon.
 13. The process of claim 9 wherein saidpowder comprises from about 50 to about 70 weight percent chromium, fromabout 12 to about 30 weight percent molybdenum, from about 15 to about25 weight percent nickel, and from about 3.5 to about 6.5 weight percentcarbon.
 14. The process of claim 9 wherein said powder comprises powderparticles, said powder particles comprising bimetallic carbides ofchromium and molybdenum dispersed in a matrix metal, said matrix metalbased on nickel/chromium/molybdenum, said powder particles having anaverage particle size (agglomerated) of from about 5 micrometers toabout 55 micrometers.
 15. The process of claim 9 wherein said powdercomprises powder particles, said powder particles comprising bimetalliccarbides of chromium and molybdenum dispersed in a matrix metal, saidmatrix metal based on nickel/chromium/molybdenum, said powder particleshaving an average particle size (agglomerated) of from about 15micrometers to about 50 micrometers.
 16. The process of claim 9 whereinsaid bimetallic carbides of chromium and molybdenum have a particle orgrain size of less than about 8 microns.
 17. The process of claim 9wherein said bimetallic carbides of chromium and molybdenum have aparticle or grain size of less than about 4 microns.
 18. The process ofclaim 9 wherein said thermal spray device is selected from detonationgun, high velocity oxygen fuel (HVOF) and plasma.